help on why 2002 impala police car appears to eat so much gas?

I found out from fueleconomy.com that this police car should get 16 mpg. It really does not appear to be doing so. I calculated that with roughly a 17 gallon tank and with a supposedly 16 mpg, i should be getting bout 272 milesfrom full to empty. Ok so i filled my tank and it took bout 45 miles before the gauge finally moved pass the full mark. As i kept driving it slowly move to the halfway mark but still looking on pace for my 272 miles. Anyway once it got to the half mark i had bout 140 miles then the gauge jus started dropping steadily fast. I didnt even go a good 4 miles when the needle dropped two lines away from the half mark. it just keeps dropping. Well i at 190miles and almost close to the low fuel light to coming on . I know i aint getting my 16mpg or prob not 15mpg. what could be wrong ? Why it appears to guzzle so much after half mark? The car is in good shape and run good and even with police package still should not be doing this. PLEASE HELP thank